1. Field of the Invention
The present invention relates to a display device having a display panel formed of a pair of substrates and a display material, such as liquid crystal, sandwiched therebetween.
2. Description of the Background Art
In a display panel (liquid-crystal panel) of a liquid-crystal display device, a liquid-crystal material is sandwiched between a pair of substrates, and a plurality of signal interconnections and scanning interconnections, perpendicular to each other, are provided on the panel. In the display panel, pixels are formed at the intersections of the signal interconnections and the scanning interconnections.
For example, in an active-matrix display panel in which each pixel has a switching element (also called “an active component”) such as a thin film transistor (TFT), the signal interconnections, scanning interconnections, and switching elements are formed on one of the two substrates. Pixel electrodes are provided opposite the respective switching elements in the pixels. The signal interconnections and scanning interconnections are controlled by an external driver device, whereby the individual switching elements are driven to turn on/off desired pixels to display images.
The manufacture of such a display device includes a cell fabrication process in which two substrates are assembled together and liquid crystal is injected therebetween, and the cell fabrication process includes rubbing processing in which polyimide films (alignment layers) applied on the substrates are rubbed with cloth in order to align the liquid-crystal material. This process causes so large amounts of static electricity that overvoltage may be applied to the scanning interconnections and signal interconnections, which may lead to breakage of switching elements and hence occurrence of line defects.
In a common method for preventing formation of such line defects, a shorting interconnection is provided to short-circuit all signal interconnections and scanning interconnections to provide increased electric capacitance, thereby dispersing and reducing the damage to the switching elements. The shorting interconnection is removed immediately before a process like inspection of images on the display panel that requires removal of the shorting interconnection. Accordingly, the shorting interconnection is placed near edges of the substrate (near edges of the display panel) so that it can be removed easily. The shorting interconnection can thus be removed easily by the beveling of the edges.
However, in the beveling process, the shorting interconnection may be insufficiently removed because of variations of the amount of beveling. Then, fragments of the shorting interconnection remaining unremoved will cause short-circuits between adjacent interconnections, leading to display defects like line defects and breakage of driver circuitry used for inspection. Such insufficient removal of the shorting interconnection can be prevented by removing larger portions by the beveling, but this leads to other problems. For example, the beveling process will then take a longer time and require increased costs, or excessive beveling may erroneously remove connecting terminals connected to the signal interconnections and scanning interconnections.
There are strong demands for weight reduction and reduction of picture-frame size of liquid-crystal display devices because they are applied to notebook personal computers, portable terminals, and the like. In an approach for meeting the demands, the area for beveling is reduced by reducing the amount of beveling of display panel edges. However, this approach needs more enhanced beveling precision. Enhancing the beveling precision without extra costs requires establishing process control capable of more precisely controlling the amount of beveling in ordinary manufacturing process.
In a suggestion made to solve this problem, marks are provided near substrate edges to be beveled in order to show the extent of beveling (for example, see Japanese Patent Application Laid-Open No. 2001-166324, which is hereinafter referred to as Patent Document 1). According to Patent Document 1, marks indicating the extent of beveling are provided near edges of a display panel in positions corresponding to a necessary minimum area including the shorting interconnection to be beveled (lower limit position), positions corresponding to a maximum area that can be beveled (upper limit position), and intermediate positions therebetween (central position). Then, the beveling process is conducted while visually checking the marks, making it possible to easily prevent insufficient removal of the shorting interconnection and excessive beveling. Also, by checking the amount of beveling with the marks, it is possible to easily grasp how the amount of beveling varies, whereby the beveling precision can be enhanced.
In particular, Patent Document 1 suggests formation of the mark and interconnection as one piece (see FIGS. 6 to 8 of Patent Document 1).
In such a conventional display panel as described in Patent Document 1, the marks are provided only in positions corresponding to the upper limit and lower limit of beveling and the central position between them. Accordingly, it is possible to know whether the amount of beveling is within a specified range (i.e., whether the finish of beveling is between the upper limit and lower limit positions), but its variation within the specified range cannot be measured. Therefore, precisely controlling the amounts of beveling further requires measuring the amounts of beveling with a precision distance meter. This results in increased process control costs and leads to increased display device manufacturing costs.